Abstract

We demonstrate three-dimensional (3D) optical trapping of subwavelength polystyrene beads and bacteria with a surface plasmonic lens fabricated on the endface of an optical fiber. To the best of our knowledge, this is the first demonstration of 3D trapping of subwavelength particles with single fiber optical tweezers. The optical power for achieving a stable 3D trap is smaller compared with conventional optical tweezers, indicating a stronger trap. Compared with surface plasmon tweezers, the trap enabled by our fiber tweezers is located ≈6 wavelengths away from the fiber endface, reducing thermal effects due to the metal absorption and preventing physical contact with the trapped objects.

Figures (4)

(a) Schematic of a submicrometer sized bead trapped by the subwavelength focus beyond the near field created by the fiber-based SP lens. Inset shows the close-up of the trap. (b) SEM image of a fiber SP lens with four concentric ring slits. (c) Cross-section view of the SP lens on the fiber end. Light guided in the fiber core is coupled to the SP modes (green arrows) in the concentric nanoscale slits in the gold coating. The transmitted light creates a focus around 4 wavelengths away in water with weak on-axis light components, reducing the scattering force to facilitate a stronger trap.

Fluorescence images of a 520 nm fluorescent polystyrene bead trapped in 3D (labeled by “T”) by the fiber-based SP lens (Media 1). A reference bead (labeled by “R”) is attached to the coverglass. The next movement is labeled at the bottom of each image. With the bead trapped, the coverglass was moved along +y (a), (b), −x, and −y (b), (c). The fiber as well as the trap was then lifted up along (c), (d) +z, followed by the coverglass movements along (d), (e) +y and (e), (f) −y. (f), (g) The bead remained trapped during these movements and could be seen when the focal plane was lifted up to the trap level. (h) The schematic of the experimental setup. The optical power at the fiber endface was 1.5 mW.

Images of 3D trapping of a bacterium with the fiber SP lens (Media 2). The white arrows indicate the bacterium, and the black arrows points to a reference silica bead. (a), (b) A free bacterium was trapped. (c)–(e) The bacterium was lifted up in the vertical direction while the focal plane was on the coverglass. (e), (f) The focal plane was brought to the plane where the bacterium was located, while the reference bead became out of focus. (f)–(h) The water was moved along +y and +x while the bacterium remained trapped. The optical power at the fiber end face was 0.9 mW.

Force mapping in the xz plane (left) and yz plane (right) obtained with the fiber SP lens. The fiber end is located at z=0 and facing downward. The particle is a sphere with a diameter of 200 nm and a refractive index of 1.38. The optical power is 10 mW.